Explain the evidence from ribosomal RNA (rRNA) sequence comparisons that led to the proposal of the three-domain system of classification.

IB DP Biology Standard Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Carl Woese proposed a radical revision of the classification of life in the 1970s based on comparisons of ribosomal RNA sequences. This led to the current three-domain system: Bacteria, Archaea, and Eukarya.

Model answer (4 marks)

rRNA genes are highly conserved, so their sequences can be aligned across all organisms to infer evolutionary relationships.

Comparing 16S/18S rRNA sequences revealed that the bacterial and archaeal sequences are as different from each other as either is from eukaryotic sequences, indicating three distinct lineages.

This contradicted earlier two‑kingdom (prokaryote/eukaryote) or five‑kingdom models that treated all prokaryotes as a single group.

Furthermore, archaeal rRNA shares several features (e.g. certain nucleotide motifs) with eukaryotic rRNA, suggesting a closer relationship between Archaea and Eukarya and supporting the three‑domain system.

Examiner tips

  • Use the word ‘evidence’ and cite rRNA conservation, sequence divergence, and shared motifs; keep each point brief and to the point.
  • Show the logical link: conserved rRNA → comparable sequences → divergence patterns → new domains.
  • Mention the contradiction to earlier models explicitly.
  • Use correct terminology: 16S, 18S, conserved, divergent, domains.

Common mistakes

  • Confusing 16S with 18S or vice versa; students often mix up bacterial and eukaryotic rRNA names.
  • Failing to state that Archaea and Bacteria are equally distant from Eukarya; they may say one is closer.
  • Omitting the point that the evidence overturned the two‑kingdom model.

Mark scheme (4 marks)

  1. rRNA sequences are highly conserved across all life and can be compared between organisms to determine evolutionary relatedness
  2. rRNA sequence comparisons showed that Archaea and Bacteria are as distantly related to each other as either is to Eukarya
  3. This evidence contradicted the previous two-kingdom or five-kingdom systems which grouped all prokaryotes together
  4. rRNA evidence also showed Archaea share some molecular features with Eukarya, supporting the three-domain arrangement where Archaea and Eukarya share a more recent common ancestor

Key terms in this question

ribosomal RNA (rRNA) · three-domain system

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